1. Field of the Invention
The present invention relates generally to servo control systems and more particularly to a nonlinear, adaptive position control system particularly suited for use in multi-axis control systems having dual mode servo subsystems for energizing a single drive mechanism per axis to effect precision positioning of a work piece relative to a work apparatus and/or optical device.
2. Description of the Prior Art
Although numerous analog control systems and techniques are known in the prior art, there is a class of adaptive position servo systems in which the overall positional accuracy and the required wide dynamic range of travel do not lend themselves readily to a purely analog solution. For example, very few analog reference devices can achieve a linearity in excess of one part in 10.sup.6.
Even though others have used laser distance measuring interferometer techniques to accomplish such accuracies, these systems have typically required the use of two distinct drive mechanisms. For example, as disclosed in the U.S. patents of Hassan et al, U.S. Pat. Nos. 3,904,945 and 4,016,396, a coarse positioning stage is used in combination with a fine positioning stage. More specifically, the coarse positioning stage is used to position the apparatus in the vicinity of the desired position and is latched in such position while the fine positioning stage, which has a much smaller range of travel, is used to make the final precise movement to the desired location. Obviously, such systems involve a substantial degree of mechanical complexity which could be reduced considerably if a single mechanical drive stage per dimensional degree of freedom were required to perform both the coarse and precision positioning operations.